The present invention relates to solid-state lasers and specifically to a method of fabricating a quantum-well photoelectric device, for example, a quantum cascade laser or quantum cascade photodetector.
Quantum cascade lasers emit light when electrons (holes) cascade through a series of quantum wells positioned between electrodes of the laser. The electrodes create an energy gradient among successive quantum wells, and within a quantum well, transient confinement of electrons (holes) splits the conduction (valence) band of the semiconductor into subbands. When the electrons (holes) pass between the subbands, stimulated emission of photons may occur. After the transition between subbands, the electrons (holes) may tunnel to an adjacent quantum well and a lower (higher) subband.
Each quantum well is defined by a thin layer of semiconducting material flanked by barrier materials whose conduction (valence) band is offset to a higher (lower) energy level. Current quantum cascade lasers are typically fabricated of GaAs and AlGaAs where the AlGaAs provides the barrier layer. During fabrication, successive layers of GaAs and AlGaAs are deposited using standard integrated-circuit deposition techniques such as chemical vapor deposition or physical vapor deposition.
Quantum cascade lasers may also be made out of alternating Si/SiGe alloy layers. These devices are typically made with SiGe alloy wells of approximately 70% Si with barrier layers of pure Si and generate emission via the movement of holes. Recently, some have suggested using SiGe alloy wells of approximately 80% Ge with Ge barriers for emission by electron transitions.
Quantum cascade lasers with significant power may require many defect-free layers of semiconductor and barrier material. Defects are imperfections in the crystal structure that adversely affect movement of electrons or holes through the device. Defects can be created when too much strain builds up within the multiple layers and a layer “relaxes” by moving atoms out of the ideal crystallographic positions.
The build up of strain in the multilayer structure is caused by the different lattice constants of the materials. For example, the different layers of a device employing Si/SiGe will be strained because the Ge atom is bigger than the Si atom and thus the Ge atoms get forced into a smaller volume available in a predominantly Si structure.
The technique of strain-symmetrization is often used to control internal stress. Instead of putting all of the strain in the SiGe alloy layers, for example, some of the strain is put into the intermediate Si (barrier) layers. This is done by starting with a “virtual substrate” with a lattice constant between that of Si and SiGe. This virtual substrate is grown such that the Si layers are stretched ((tensilely strained) and the SiGe alloy layers are compressed. The remaining layers are grown on this virtual substrate, which provides a compromise lattice constant ideally minimizing strain within the subsequent layers.
One way to create a virtual substrate is to grow a SiGe alloy on a bulk Si substrate. By gradually increasing the Ge concentration, the desired lattice constant is reached. Despite best efforts, however, such virtual substrates have surface defects and further surface roughness/waviness that eventually cause problems in the subsequently deposited layers. As a result, the crystallographic quality steadily degrades as the structure is grown and before a useful device is achieved.